Angular momentum conservation

A person stands on a platform, initially at rest, that can rotate freely without friction. The moment of inertia of the person plus the platform is IP. The person holds a spinning bicycle wheel with its axis horizontal. The wheel has a moment of inertia Iw and angular velocity WW. What will be the angular velocity WP of the platform if the person moves the axis of the wheel so that it points vertically upward?

2. Relevant equations

L_initial = L_final

3. The attempt at a solution

I didn't understand why the angular momentum is conserved only in vertical axis. There is no external force, so what has happened to angular momentum in the horizontal? Wıth regard to what should I choose an axis so that I can apply the conservation?